My dictionary does not even have the word Kärtchen, and online translators have very little about this noun. I think that it's neuter (das), yet I would like to know if this word has a plural form and if so, how it is spelled.

2 Answers
2

One reason you are having difficulties finding an adequate dictionary entry is because Kärtchen is what is known as a diminutive form. One of the diminutive endings -chen or -lein, when added to nouns, makes the modified noun smaller, cuter, or (less often) laughable. (Unfortunately for many words one of the two diminutives is much more common than the other, but there is no easy rule for it.)

In the case of Kärtchen, this is the diminutive form of die Karte. As you can see, some vowels (a, o, and u) become the umlauted diphthongs (ä, ö, ü) when the diminutive is made. Additionally, nouns ending in -e often (but not always) drop this letter. Both of the above morphological phenomena can be seen in the example of die Karte -> das Kärtchen.

Following the above rule regarding the pluralization of diminutives, its plural is die Kärtchen. Since every noun has a possible diminutive form, most are left out of the dictionary. The only ones that are found in the dictionary are typically ones with concrete meanings such as Brötchen, Mädchen (girl), etc.

Note:
Not every word that ends in the letters chen is a diminutive variant, as Alexander has pointed out in the comments. A few examples: